A variety of optical communications modules are used in optical networks for transmitting and receiving optical data signals over the networks. An optical communications module may be an optical receiver module that has optical receiving capability, but not optical transmitting capability. Alternatively, an optical communications module may be an optical transmitter module that has optical transmitting capability, but not optical receiving capability. Alternatively, an optical communications module may be an optical transceiver module that has both optical transmitting and optical receiving capability.
A typical optical transmitter or transceiver module has a transmit (Tx) portion that includes a laser driver circuit and at least one laser diode. The laser driver circuit outputs an electrical drive signal to each respective laser diode to cause the respective laser diode to be modulated. When the laser diode is modulated, it outputs optical signals that have power levels corresponding to logic 1s and logic 0s. An optics system of the module focuses the optical signals produced by each respective laser diode into the end of a respective transmit optical fiber held within an optical connector module that connects to the optical transmitter or transceiver module.
A typical optical receiver or transceiver module has a receive (Rx) portion that includes at least one receive photodiode that receives an incoming optical signal output from the end of a respective receive optical fiber held in an optical connector module. The optics system of the receiver or transceiver module focuses the light that is output from the end of each receive optical fiber onto the respective receive photodiode. The respective receive photodiode converts the incoming optical signal into an electrical analog signal. An electrical detection circuit, such as a transimpedance amplifier (TIA), receives the electrical signal produced by the receive photodiode and outputs a corresponding amplified electrical signal, which is processed by other circuitry of the Rx portion to recover the data.
Some optical transceiver modules have a single laser diode in the Tx portion and a single photodiode in the Rx portion for simultaneously transmitting and receiving optical signals over transmit and receive fibers, respectively, of transmit and receive optical cables, respectively. The ends of the transmit and receive cables have optical connector modules on them that are adapted to plug into transmit and receive receptacles, respectively, formed in the optical communications module. These types of optical communications modules are often referred to as pluggable modules. Small form-factor pluggable (SFP) and SFP+ communications modules are examples of pluggable optical communications modules.
Some optical communications modules have multiple laser diodes and/or multiple photodiodes for simultaneously transmitting and/or receiving multiple optical signals. In these types of optical modules, which are commonly referred to as parallel optical modules, the transmit fiber cables and the receive fiber cables have multiple transmit optical fibers and multiple receive optical fibers, respectively. The cables are typically ribbon cables having ends that are terminated in an optical connector module that is configured to be plugged into a receptacle of the parallel optical communications module.
The Federal Communications Commission (FCC) has set standards that limit the amount of electromagnetic radiation that may emanate from unintended sources. For this reason, a variety of techniques and designs are used to shield EMI open apertures in optical communications module housings in order to limit the amount of EMI that passes through the apertures. Various metal shielding designs and resins that contain metallic material have been used to cover areas from which EMI may escape from the housings. So far, such techniques and designs have had only limited success, especially with respect to parallel optical communications modules that transmit and/or receive data at high data rates (e.g., 10 gigabits per second (Gbps) and higher) over multiple parallel channels.
The amount of EMI that passes through an EMI shielding device is proportional to the largest dimension of the largest EMI open aperture of the EMI shielding device. Therefore, EMI shielding devices such as EMI collars and other devices are designed to ensure that there is no open aperture that has a dimension that exceeds the maximum allowable EMI open aperture dimension associated with the frequency of interest.
In parallel optical transceiver modules, the optical cables that carry the fibers are typically ribbon cables in which the fibers are arranged side-by-side in a 1×N array, where N is the number of fibers of the ribbon cable. Thus, the transmit fibers are arranged in one 1×N fiber array in one ribbon cable and the receive fibers are arranged in another 1×N array in another ribbon cable. Typically, the ribbon cables are placed one on top of the other such that a 2×N array of fibers enter the optical connector module through a gap formed in the nose of the optical connector module. This gap constitutes an EMI open aperture that is much larger than the maximum allowable EMI open aperture dimension of the optical transceiver module, particularly at high data rates. Consequently, unacceptable amounts of EMI may escape from the optical transceiver module through the gap.
Accordingly, a need exists for an EMI shielding device and a method that provide effective EMI attenuation at the gap in the optical communications module through which the optical fiber ribbon cables pass.